Billy Joel Hires Bodyguard for Daughter Alexa Ray

Alo Ceballos/FilmMagic(NEW YORK) -- Pop legend Billy Joel reportedly hired a private investigator to track down a woman accused of stalking his daughter for months and writing her frightening messages on Facebook.

The “Piano Man" songwriter hired a private security firm to find the woman, according to the New York Post. The security firm was reportedly recommended to Joel by none other than Beatles superstar Paul McCartney.

The alleged stalker, 40-year-old Sheryl Finley, was arrested in January, according to the Post. Citing court records, the paper said she was found naked in the woods in Minnesota. The Post added that she had spent months in a mental institution in 2010, and wrote several Facebook messages referencing pedophilia, sadistic-sexual behavior, violent physical assaults and murder.

The messages were allegedly sent from a fake Facebook account under the name “Rick Steenfield.” Alexa Ray Joel, who is also a songwriter, was reportedly afraid to leave her Manhattan home.

"We need to take the harassment and the threats that occur, via those digital means, very seriously," Austin Police Department Capt. Dave McKichan told ABC News affiliate KAAL-TV in Texas.

According to the British newspaper The Daily Telegraph, Finley was charged with a felony for stalking Joel through electronic messages sent by false impersonation, and with a gross misdemeanor for stalking via repeated electronic messages.

She had pleaded not guilty to the charges, and has a pre-trial hearing coming up in a few weeks.

Brad Garrett, ABC News consultant and former FBI agent, said many celebrity stalkers are driven by delusion.

"Delusion that they either have a relationship with the famous person. Or they desire one....You have to separate the vile, violent words to the ability of the person or even the motivation of the person to actually carry them out," he said.